The older patent application DE 10 2012 209 410.5 discloses a method for determining an individual patient-specific contrast medium impulse response function (known hereinafter, for short, as “impulse response function” or “patient function”) based on test bolus data, with which a prediction of the contrast medium behavior can later be created in the context of a contrast medium-assisted imaging measurement.
This patient function describes the cardiovascular properties of the patient at the time point when the test bolus was measured. In principle, the contrast medium behavior could be predicted for any desired injection protocol under the assumption that the individual patient function is still valid at the later time point. An injection protocol is a precise time-related stipulation according to which the quantity of contrast medium is administered to the patient and includes, for example, the starting time point, the flow rate and the end time point of the contrast medium administration.
Algorithms for this purpose, referred to as contrast enhancement prediction (CEP) algorithms, are generally known from a variety of publications such as, for example:                Dominik Fleischmann et al., “Mathematical Analysis of Arterial Enhancement and Optimization of Bolus Geometry for CT Angiography Using the Discrete Fourier Transform”, Journal of Computer Assisted Tomography, 1999, vol. 23, No. 3, pages 474 to 484, and        Andreas H. Mahnken et al., “Quantitative prediction of contrast enhancement from test bolus data in cardiac MSCT”, Eur. Radiol. 2007, 17, pages 1310 to 1319.        
These CEP algorithms calculate the likely behavior of the contrast medium in the patient and are usually constructed from two sub-algorithms. A first sub-algorithm determines a patient-specific contrast medium impulse response function (or arterial impulse response, AIR) and is known as the “AIR algorithm”. The second sub-algorithm calculates a prediction of the likely contrast medium behavior in the patient and is known as the “PRED algorithm”. A CEP algorithm therefore usually includes a combination of an AIR algorithm and a PRED algorithm.
In all these known methods, the patient being examined is regarded as being a linear time-invariant (LTI) system. This means, in general, that the contrast medium accumulation in the patient C(t) can be expressed mathematically as a convolution of the injection protocol IF(t) with the impulse response function or patient function AIR(t):C(t)=IF(t){circle around (x)}AIR(t)  [1]
As a consequence, the CEP algorithms carry out all the calculations in the time domain and/or the Fourier domain which is derived from the time domain.
However, this is based on the assumption that the driving force for the contrast medium, the blood circulation, is invariable and stable over time. However, this is often not the case.
A typical area of application for contrast medium-assisted imaging methods is, for example, CT angiography (CTA), that is, the imaging of the vessels by way of computed tomography. A CTA scan is an operation of a computed tomography scanner to image the vessels, and a heart CTA or heart scan is the imaging of the heart and representation of the heart vessels.
It can occur, in heart scans during CT angiography (CTA), in particular, that the heart rate (number of heartbeats per time interval) is lastingly raised or lowered following the test bolus, for example, under the influence of beta blockers (e.g. metoprolol), which are typically administered shortly before the scan to patients with a raised heart rate. Since the recirculation time of the blood changes at a different heart frequency, the patient function also changes in this case. This would lead to an imprecise prediction of the contrast medium behavior.